A high-resolution process study of High Salinity Shelf Water formation in the Terra Nova Bay Polynya, Ross Sea, Antarctica

Polynyas are large, seasonal openings in sea ice that allow for the transfer of gas, heat, and momentum between the atmosphere and an ocean otherwise sealed off by ice. Many occur along the coasts of Antarctica, where katabatic winds push ice seaward, allowing new ice to form. The ongoing brine rejection as well as exposure to the cold atmosphere increases the density of surface water within the polynya, forming a water mass known as High Salinity Shelf Water (HSSW). HSSW sinks and mixes with other water masses to eventually become Antarctic Bottom Water, a water mass that drives the overturning of ocean nutrients, carbon, and heat on a global scale. The preconditioning and formation mechanisms that control HSSW production are not well constrained, highlighting the need for comprehensive and high-resolution observational studies of this complex Antarctic air-ice-ocean system. Here, we utilize measurements of salinity, temperature, water velocity, ocean surface waves, and sea ice thickness from a mooring deployed in the Terra Nova Bay polynya from February 2017 through February 2018 in order to investigate HSSW on timescales ranging from sub-hourly to interseasonal. The mooring was uniquely instrumented throughout the water column to capture the processes surrounding HSSW formation and transport at high temporal resolution, and is unprecedented in its in-situ measurements of the near surface (~50m water depth) during the austral winter, when HSSW formation occurs. It was located in the direct path of katabatic winds (via the Reeves Glacier) and in line with the Automatic Weather Station Manuela, which provides key measurements of wind speed and other meteorological variables. We identify polynya opening events using both in-situ measurements of sea ice thickness and MODIS infrared and AMSR2 passive microwave satellite imagery, and present selected events as case studies to illustrate the mechanisms and response times of HSSW production. We compare these opening events to characterize how HSSW production evolves over the course of the year and additionally test the ability of mixed layer models to capture the observed HSSW properties and response times.